Among the various video display systems available in the art, an optical projection system is known to be capable of providing high quality displays in a large scale. In such an optical projection system, light from a lamp is uniformly illuminated onto an array of, e.g., M.times.N, actuated mirrors, wherein each of the mirrors is coupled with each of the actuators. The actuators may be made of an electrodisplacive material such as a piezoelectric or an electrostrictive material which deforms in response to an electric field applied thereto.
The reflected light beam from each of the mirrors is incident upon an aperture of, e.g., an optical baffle. By applying an electrical signal to each of the actuators, the relative position of each of the mirrors to the incident light beam is altered, thereby causing a deviation in the optical path of the reflected beam from each of the mirrors. As the optical path of each of the reflected beams is varied, the amount of light reflected from each of the mirrors which passes through the aperture is changed, thereby modulating the intensity of the beam. The modulated beams through the aperture are transmitted onto a projection screen via an appropriate optical device such as a projection lens, to thereby display an image thereon.
In FIGS. 1A to 1F, there are illustrated manufacturing steps involved in manufacturing an array 100 of M.times.N thin film actuated mirrors 101, wherein M and N are integers, disclosed in a copending commonly owned application, U.S. Ser. No. 08/589,478, entitled "METHOD FOR FORMING A THIN FILM ACTUATED MIRROR ARRAY".
The process for the manufacture of the array 100 begins with the preparation of an active matrix 110 including a substrate 112 with an array of M.times.N connecting terminals 114 and an array of M.times.N transistors(not shown), wherein each of the connecting terminals 114 is electrically connected to a corresponding transistor in the array of transistors.
In a subsequent step, a thin film sacrificial layer 120, made of phosphor-silicate glass (PSG), and having a thickness of 0.1-2 .mu.m, is formed on top of the active matrix 110 by using a chemical vapor deposition(CVD) method or a spin coating method.
Thereafter, there is formed an array of M.times.N pairs of empty cavities (not shown) in the thin film sacrificial layer 120 by using an wet etching method. One of the empty cavities in each pair encompasses one of conduits 126.
Subsequently, an elastic layer 130, made of an insulating material, e.g., silicon nitride, and having a thickness of 0.1-2 .mu.m, is deposited on top of the thin film sacrificial layer 120 including the empty cavities by using a CVD method.
Thereafter, there is formed in the elastic layer 130 an array of M.times.N conduits 126 made of a metal, e.g., tungsten(W). Each of the conduits 126 is formed by: first creating an array of M.times.N holes(not shown), each of the holes extending from top of the elastic layer 130 to top of a corresponding connecting terminal 114 by using an etching method; and filling therein with the metal by using, e.g., a lift-off method, as shown in FIG. 1A.
Then, a second thin film layer 140, made of an electrically conducting material, e.g., aluminum(Al), and having a thickness of 0.1-2 .mu.m, is formed on top of the elastic layer 130 including the conduits 126 by using a sputtering or a vacuum evaporation method.
Next, a thin film electrodisplacive layer 150, made of a piezoelectric material, e.g., lead zirconium titanate(PZT), or an electrostrictive material, e.g., lead magnesium niobate(PMN), and having a thickness of 0.1-2 .mu.m, is formed on top of the second thin film layer 140 by using a CVD method, an evaporation method, a Sol-Gel method or a sputtering method.
In an ensuing step, a first thin film layer 160, made of an electrically conducting and light reflecting material, e.g., aluminum(Al) or silver(Ag), and having a thickness of 0.1-2 .mu.m, is formed on top of the thin film electrodisplacive layer 150 by using a sputtering or a vacuum evaporation method, as shown in FIG. 1B.
After the above step, the first thin film 160, the thin film electrodisplacive 150, the second thin film 140 and the elastic layers 130 are patterned into an array of M.times.N first thin film electrodes 165, an array of M.times.N thin film electrodisplacive members 155, an array of M.times.N second thin film electrodes 145 and an array of M.times.N elastic members 135 by using a photolithography or a laser trimming method, thereby forming an array 180 of M.times.N actuated mirror structures 181, as shown in FIG. 1C. Each of the second thin film electrodes 145 is electrically connected to a corresponding connecting terminal 114 through the conduit 126, thereby functioning as a signal electrode in each of the thin film actuated mirrors 101. Each of the first thin film electrodes 165 functions as a mirror as well as a bias electrode therein.
The preceeding step is then followed by completely covering the top surface and the side surfaces in each of the actuated mirror structures 181 with a thin film protection layer 170, as shown in FIG. 1D.
The thin film sacrificial layer 120 is then removed by using an etchant, e.g., hydrogen fluoride(HF), as shown in FIG. 1E.
Finally, the thin film protection layer 170 is removed by using an etching method, e.g., a plasma etching method thereby forming the array 100 of M.times.N thin film actuated mirrors 101, as shown in FIG. 1F.
There are certain deficiencies associated with the above described method for manufacturing the array 100 of M.times.N thin film actuated mirrors 101. Since the thin film sacrificial layer 120 is made of a phosphor-silicate glass (PSG), during the formation of empty cavities by using an wet etching method, phosphorus on the surface of thin film sacrificial layer 120 are oxidized into phosphorus pentoxides (P.sub.2 O.sub.5) decreasing the adhensivity between the thin film sacrificial layer 120 and a photoresist used in the wet etching method, in such a way that the thin film sacrificial layer 120 located under the unexposed portions of the photoresist may get wrongfully removed.
Furthermore, the phosphorus pentoxides (P.sub.2 O.sub.5) formed on the surface of the thin film sacrificial layer 120 may react with moisture to form phosphoric acid (H.sub.3 PO.sub.4), damaging the thin film sacrificial layer.